Carver 3608 running speed

My 1994 370 (same boat) with 454's runs 19-20 mph at 32-3300 rpm and tops out at 32/33 mph at 4400 rpm.

I'm a new boater so please don't mistake this question for arrogance....I am sincerely asking this in the hope of learning the right way to ask the "speed question"

My marine mechanic, who owns a successful firm that has serviced many boats in my area for many years, told me when I first questioned him about my boat's speed that experienced boaters NEVER talk in "Miles per hour", since that phrase is "meaningless" (his words not mine) on the water. It's "knots" and "nautical miles" according to him.

So is it ok to use "MPH" when talking about speed of a boat or not?

What's the right way?

In my opinion there is no right or wrong way, I simply have this burned into my brain from my GPS reading MPH.

This will cover everyone, I cruise at 17 knots and top out at 28 knots.

You really have two speeds that are best. You're optimal fast cruising speed which will give you best speed with least fuel burn. This is generally a few hundred RPM higher than planning speed where the boat first gets on step.

You also have hull speed which you can calculate yourself. 1.34x the square root of the water line. Without looking up your specs, it's probably about 8 knots. These two speeds will be your boat's most efficient. Any other and you're wasting fuel.